1. Field of the Invention
This invention relates to a noncontact tonometer comprising position detecting means for detecting a position of an apparatus body of the tonometer with respect to a subject's eye.
2. Description of the Related Art
Conventionally, a noncontact tonometer has been known which deforms (i.e., flattens) the cornea C of a subject's eye E by spraying a fluid, such as an air pulse, from a spraying nozzle N, such as that shown in FIG. 9, onto the cornea C, and measures the intraocular pressure of the eye by receiving rays of light reflected by the flattened cornea C with a light receiving element. In the noncontact tonometer, a positional adjustment of the spraying nozzle N to the cornea C needs to be made, and thus the noncontact tonometer includes alignment detecting means for detecting a relative position (hereinafter, referred to as alignment) of its apparatus body to the eye E. And, an alignment detecting method has been known which comprises steps of projecting an index from front onto the cornea C, then receiving rays of light reflected by the corneal surface, and detecting an alignment state from a position of the reflected image.
However, the noncontact tonometer includes a spraying nozzle N for spraying a fluid which is disposed in front of the cornea C. Thus, an index for detecting alignment needs to be projected through the nozzle. In addition, the diameter of a beam of light to be projected needs to be made as large as possible so that a larger alignment-detectable area can be obtained. Hereinafter, a relationship between the subject's eye E and the diameter of the beam will be described in detail hereinafter with reference to FIG. 10, but the spraying nozzle N is not shown in FIG. 10 for convenience of explanation.
FIGS. 10(a) to 10(c) show an alignment optical system 200. The alignment optical system 200 comprises a half mirror 201, an objective lens 202, and a PSD 203. FIG. 10(a) shows a state where a positional adjustment has been made between the cornea C and the apparatus body; FIG. 10(b) shows a state where disalignment has occurred between a vertex P of the cornea C and the apparatus body; and FIG. 10(c) shows a state where disalignment has occurred between the vertex P of the cornea C and the apparatus body and, simultaneously, the diameter of an index beam of light is small.
As shown in FIGS. 10(a) to 10(c), an index beam of light K, which has been collimated by an index beam projecting optical system (not shown) for projecting an index beam of light, is reflected by the half mirror 201, and then is guided to a surface T of the cornea C. At this time, if the alignment has already been completed, the index beam K projected onto the cornea C is reflected by the surface T so as to form a luminous image R at a position between a vertex P of the cornea C and the curvature center of the cornea C. The reflected index beam passes through the half mirror 201, is then condensed by the objective lens 202, and forms an image R' opposite to the luminous image R at the center of the PSD 203.
In the case where the diameter of the index beam K is sufficiently large (see FIGS. 10(a), 10(b)), if there is no disalignment between the cornea C and the apparatus body, as a matter of course, the beam reflected by the cornea C forms an image on the PSD 203 through the objective lens 202. Even if there is disalignment therebetween, a part of the beam forms an image thereon by returning to its optical path, so that an alignment state can be detected. On the other hand, in the case where the diameter of the index beam K is small (see FIG. 10(c)), if its relative position is largely shifted, the beam reflected by the cornea C is reflected in a direction deviated from its optical path, and thus cannot impinge the objective lens 202. Therefore, the reflected beam does not reach the PSD 203, and an alignment state cannot be detected. In other words, it is desired that the index beam K having a larger width is used which enlarges an alignment detectable area.
Therefore, as shown in FIG. 9, if the beam K passes through the whole area of the inside of the nozzle N, a beam of light K1 out of the optical axis will strike an end surface F of the nozzle N on the side of the objective lens, and the reflected (scattering) light K1' from the end surface F will enter the light receiving element of a detecting optical system. If an element for detecting the centroid position of incident light, such as the PSD 203 shown in FIG. 10, is used as a light receiving element, the beam reflected by the end surface F of the nozzle N acts as a noise which causes the false detection of alignment.